Object of class stdClass could not be converted to string

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /admin/structure/views/view/follow_node_updates/preview/page_1/ajax
StatusText: Service unavailable (with message)
ResponseText: Recoverable fatal error: Object of class stdClass could not be converted to string in DatabaseStatementBase->execute() (line 2168 of /var/www/includes/database/database.inc).

This happens when I tried the following field output code.

$tmp_user=user_load($data->users_flag_content_uid);
$data->is_member=false;
foreach ($data->field_og_group_ref as $tmp){ 
  if ( og_is_member( $tmp['raw']['target_id'] , 'user', $tmp_user ) ) {  
    $data->is_member = true;
    break;
  }
}
dpm ($data);